In order to be able to multiply matrices together, the columns of the first matrix HAS to equal the rows of the second matrix.

Since 3 does not equal 2, it can not be multiplied. Note, too, that AB does not equal BA.

One more useful tool is if the columns of the first does equal the rows of the second, that is, you can multiply the matrices, the answer will the number of rows the first matrix has by the number of columns the second matrix has.